Generally, an absolute distance meter (ADM) is a device that determines the distance to a remote target. It does this by sending laser light to the target and then collecting light that the target reflects or scatters. An ADM may be used to measure distances in one dimension, as might be seen, for example, in a consumer product available at a hardware store. It may be attached into a more complex device having the ability to measure quantities corresponding to additional dimensions (degrees of freedom).
An example of a device of the latter type is the laser tracker, which measures three-dimensional spatial coordinates. Exemplary systems are described by U.S. Pat. No. 4,790,651 to Brown et al. and U.S. Pat. No. 4,714,339 to Lau et al. The laser tracker sends a laser beam to a retroreflector target held against a surface of interest or placed into a fixed nest. The most common type of retroreflector target is the spherically mounted retroreflector (SMR), which may comprise a cube-corner retroreflector mounted within a sphere with the vertex of the cube-corner at the sphere center.
A device that is closely related to the laser tracker is the laser scanner. The laser scanner steps one or more laser beams to points on a diffuse surface. The laser tracker and laser scanner are both coordinate-measuring devices. It is common practice today to use the term laser tracker to also refer to laser scanner devices having distance- and angle-measuring capability. Another device closely related to the laser tracker is the total station, typically used by surveyors. The broad definition of laser tracker, which includes laser scanners and total stations, is used throughout this document.
A radar device is similar to a laser tracker in that it emits and receives electromagnetic waves and analyzes the received waves to learn the distance to a target. Radars usually emit waves in the RF, microwave, or millimeter region of the electromagnetic spectrum, whereas laser trackers usually emit waves in the visible or near-infrared region. Radars may be either bistatic or monostatic. Monostatic radars emit and receive electromagnetic energy along a common path, whereas bistatic radars emit and receive on different paths. Total stations may also be either bistatic or mono static. Laser trackers used for high accuracy industrial measurement, however, are monostatic.
To understand why laser trackers are monostatic, consider a beam emitted by the laser tracker that travels to a retroreflector target and is retroreflected back on itself. If a bistatic mode were used in the tracker, the incident laser beam would strike off the retroreflector center and the reflected laser beam would shift relative to the incident beam. Small-size retroreflector targets of the sort often used with laser trackers would not be compatible with such a bistatic device. For example, a common type of retroreflector target is the 0.5-inch diameter SMR. The cube-corner retroreflector in such an SMR typically has a clear aperture diameter of about 0.3 inch, which equals about 7.5 mm. The 1/e2 irradiance diameter of a laser beam from a tracker might be about this large or larger. Consequently, any shift in the laser beam would cause the beam to be clipped by the SMR. This would result in an unacceptably large drop in optical power returned to the tracker.
Bistatic geometry would also be problematic for a fiber-optic based ADM system. In a monostatic laser tracker that launches laser light from an optical fiber, a laser collimator can be made by placing the end face of the optical fiber at the focal point of a collimating lens. On the return path from the distant retroreflector, collimated laser light again strikes the collimating lens, although in general the returning laser beam may be off center with respect to the outgoing laser light. The fiber end face is located at the focus of the collimating lens, which has the effect of causing the light from the retroreflector target to be efficiently coupled back into the fiber, regardless of where the beam strikes the lens. In a bistatic device, alignment of the fiber-optic receiving optics is much more challenging and coupling efficiency is much lower.
One type of laser tracker contains only an interferometer (IFM) without an absolute distance meter. If an object blocks the path of the laser beam from one of these trackers, the IFM loses its distance reference. The operator must then track the retroreflector to a known location to reset to a reference distance before continuing the measurement. A way around this limitation is to put an ADM in the tracker. The ADM can measure distance in a point-and-shoot manner, as described in more detail below. Some laser trackers contain only an ADM without an interferometer. An exemplary laser tracker of this type is described in U.S. Pat. No. 5,455,670 to Payne, et al. Other laser trackers typically contain both an ADM and an interferometer. An exemplary laser tracker of this type is described in U.S. Pat. No. 5,764,360 to Meier, et al.
A gimbal mechanism within the laser tracker may be used to direct a laser beam from the tracker to the SMR. Part of the light retroreflected by the SMR enters the laser tracker and passes onto a position detector. A control system within the laser tracker can use the position of the light on the position detector to adjust the rotation angles of the mechanical azimuth and zenith axes of the laser tracker to keep the laser beam centered on the SMR. In this way, the tracker is able to follow (track) an SMR that is moved over the surface of an object of interest.
Angular encoders attached to the mechanical azimuth and zenith axes of the tracker may measure the azimuth and zenith angles of the laser beam (with respect to the tracker frame of reference). The one distance measurement and two angle measurements performed by the laser tracker are sufficient to completely specify the three-dimensional location of the SMR.
One of the main applications for laser trackers is to scan the surface features of objects to determine their geometrical characteristics. For example, an operator can determine the angle between two surfaces by scanning each of the surfaces and then fitting a geometrical plane to each. As another example, an operator can determine the center and radius of a sphere by scanning the sphere surface.
Prior to U.S. Pat. No. 7,352,446 to Bridges et al., an interferometer, rather than an ADM, was required for the laser tracker to scan moving targets. Until that time, absolute distance meters were too slow to accurately find the position of a moving target. To get full functionality with both scanning and point-and-shoot capability, early laser trackers needed both an interferometer and an ADM.
A general comparison of interferometric distance measurement and absolute distance measurement follows. In the laser tracker, an interferometer (if present) may determine the distance from a starting point to a finishing point by counting the number of increments of known length (usually the half-wavelength of the laser light) that pass as a retroreflector target is moved between the two points. If the beam is broken during the measurement, the number of counts cannot be accurately known, causing the distance information to be lost. By comparison, the ADM in a laser tracker determines the absolute distance to a retroreflector target without regard to beam breaks, which also allows switching between a plurality of targets. Because of this, the ADM is said to be capable of “point-and-shoot” measurement.
Although there are several sources of error in an interferometer measurement, in most cases the dominant error is in the value of the average wavelength of the laser light over its path through the air. The wavelength at a point in space is equal to the vacuum wavelength of the laser light divided by the index of refraction of the air at that point. The vacuum wavelength of the laser is usually known to high accuracy (better than one part in 10,000,000), but the average refractive index of air is known less accurately. The refractive index of air is found by first using sensors to measure the temperature, pressure, and humidity of the air and then inserting these measured values into an appropriate equation, such as the Ciddor equation or the Edlin equation.
However, the temperature, pressure, and humidity are not uniform over space, and neither are the sensors perfectly accurate. For example, an error in the average temperature of one degree Celsius causes an error in the refractive index of about one part per million (ppm). As mentioned above, the wavelength of light in air is inversely proportional to the air refractive index.
Similarly, in an ADM, the so-called ADM wavelength of the amplitude modulation envelope (also known as the ambiguity range) is inversely proportional to the air group refractive index. Because of this similarity, errors in measuring temperature, pressure, and humidity cause errors in calculated distance that are approximately equal for ADM and interferometer systems.
However, ADMs are prone to errors not found in interferometers. To measure distance, an interferometer uses an electrical counter to keep track of the number of times that two beams of light have gone in and out of phase. The counter is a digital device that does not have to respond to small analog differences. By comparison, ADMs are usually required to measure analog values, such as phase shift or time delay, to high precision.
In most high-performance ADMs, laser light is modulated, either by applying an electrical signal to the laser source or by sending the laser light through an external modulator such as an acousto-optic modulator or electro-optic modulator. This modulated laser light is sent out of the ADM to a remote target, which might be a retroreflector or a diffuse surface. Light reflects or scatters off the remote target and passes, at least in part, back into the ADM.
To understand the difficulties faced by ADMs, we consider two common ADM architectures: temporally incoherent architecture and temporally coherent architecture. In some temporally coherent systems, the returning laser light is mixed with laser light from another location before being sent to an optical detector that converts the light into an electrical signal. This signal is decoded to find the distance from the ADM to the remote target. In such systems, modulation may be applied to the amplitude, phase, or wavelength of the laser light. In other temporally coherent systems, several pure laser lines having different wavelengths are combined before being sent to the retroreflector. These different wavelengths of light are combined at the detector, thereby providing “synthetic” modulation.
In temporally incoherent optical systems, light is not usually mixed with light of another wavelength in an optical detector. The simplest type of temporally incoherent system uses a single measure channel and no reference channel. Usually laser light in such systems is modulated in optical power. Light returning from the retroreflector strikes an optical detector that converts the light into an electrical signal having the same modulation frequency. This signal is processed electrically to find the distance from the tracker to the target. The main shortcoming of this type of system is that variations in the response of electrical and optical components over time can cause jitter and drift in the computed distance.
To reduce these errors in a temporally incoherent system, one approach is to create a reference channel in addition to the measure channel. This is done by creating two sets of electronics. One set of electronics is in the measure channel. Modulated laser light returned from the distant retroreflector is converted by an optical detector to an electrical signal and passes through this set of electronics. The other set of electronics is in the reference channel. The electrical modulation signal is applied directly to this second set of electronics. By subtracting the distance measured in the reference channel from the distance found in the measure channel, jitter and drift are reduced in ADM readings. This type of approach removes much of the variability caused by electrical components, especially as a function of temperature. However, it cannot remove variability arising from differences in electro-optical components such as the laser and detector.
To reduce these errors further, part of the modulated laser light can be split off and sent to an optical detector in the reference channel. Most of the variations in the modulated laser light of the measure and reference channels are common mode and cancel when the reference distance is subtracted from the measure distance.
Despite these improvements, drift in such ADM systems can still be relatively large, particularly over long time spans or over large temperature changes. All of the architectures discussed above are subject to drift and repeatability errors caused by variations in optical and electrical elements that are not identical in the measure and reference channels. Optical fibers used in ADM systems change optical path length with temperature. Electrical assemblies used in ADM systems, such as amplifiers and filters, change electrical phase with temperature.
A method and apparatus for greatly reducing the effects of drift in an ADM within a laser tracker is taught in U.S. Pat. No. 6,847,436 to Bridges, the contents of which are herein incorporated by reference. This method involves use of a chopper assembly to alternately redirect returning laser light to a measure or reference path. Although this method works well, there is a limitation in the maximum rate of rotation of the chopper wheel and hence in the data collection rate of the ADM.
A method of measuring the distance to a moving retroreflector is taught in U.S. Pat. No. 7,352,446 to Bridges et al., the contents of which are herein incorporated by reference. To obtain the highest possible performance using the method of U.S. Pat. No. 7,352,446, the distances are recomputed at a high rate, preferably at a rate of at least 10 kHz. It is difficult to make a mechanical chopper as in U.S. Pat. No. 6,847,436 with a data rate this high. Hence another method needs to be found to solve the ADM drift problem.
It is possible to correct for drift in a distance meter by mechanically switching an optics beam between two free-space optical paths. One optical path, which is called the reference path, is internal to the instrument. The second optical path, which is called the measure path, travels out from the instrument to the object being measured and then back to the instrument. Light from the measure and reference paths strikes a single optical detector. Because of the action of the mechanical switch, the light from the two reference paths does not strike the single optical detector at the same time. The mechanical switch may be a mechanically actuated optical component such as a mirror, prism, beam splitter, or chopper wheel. The actuator may be a solenoid, motor, voice coil, manual adjuster, or similar device. Because the optical detector and electrical circuitry is the same for the measure and reference paths, almost all drift error is common mode and cancels out. Examples of inventions based on this method include U.S. Pat. No. 3,619,058 to Hewlett et al.; U.S. Pat. No. 3,728,025 to Madigan et al.; U.S. Pat. No. 3,740,141 to DeWitt; U.S. Pat. No. 3,779,645 to Nakazawa et al.; U.S. Pat. No. 3,813,165 to Hines et al.; U.S. Pat. No. 3,832,056 to Shipp et al.; U.S. Pat. No. 3,900,260 to Wendt; U.S. Pat. No. 3,914,052 to Wiklund; U.S. Pat. No. 4,113,381 to Epstein; U.S. Pat. No. 4,297,030 to Chaborski; U.S. Pat. No. 4,453,825 to Buck et al.; U.S. Pat. No. 5,002,388 to Ohishi et al.; U.S. Pat. No. 5,455,670 to Payne et al.; U.S. Pat. No. 5,737,068 to Kaneko et al.; U.S. Pat. No. 5,880,822 to Kubo; U.S. Pat. No. 5,886,777 to Hirunuma; U.S. Pat. No. 5,991,011 to Damm; U.S. Pat. No. 6,765,653 to Shirai et al.; U.S. Pat. No. 6,847,436 to Bridges; U.S. Pat. No. 7,095,490 to Ohtomo et al.; U.S. Pat. No. 7,196,776 to Ohtomo et al.; U.S. Pat. No. 7,224,444 to Stierle et al.; U.S. Pat. No. 7,262,863 to Schmidt et al.; U.S. Pat. No. 7,336,346 to Aoki et al.; U.S. Pat. No. 7,339,655 to Nakamura et al.; U.S. Pat. No. 7,471,377 to Liu et al.; U.S. Pat. No. 7,474,388 to Ohtomo et al.; U.S. Pat. No. 7,492,444 to Osada; U.S. Pat. No. 7,518,709 to Oishi et al.; U.S. Pat. No. 7,738,083 to Luo et al.; and U.S. Published Patent Application No. US2009/0009747 to Wolf et al. Because all of these patents use mechanical switches, which are slow, none can switch quickly enough to be used in an ADM that accurately measures a moving retroreflector.
Another possibility is to correct drift only in the electrical, and not the optical, portion of a distance meter. In this case, light from the reference optical path is sent to the reference optical detector and light from the measure optical path is sent to the measure optical detector. The electrical signals from the reference and optical detectors travel to an electrical switch, which alternately routes the electrical signals from the two detectors to a single electrical unit. The electrical unit processes the signals to find the distance to the target. Examples of inventions based on this method include: U.S. Pat. No. 3,365,717 to Hölscher; U.S. Pat. No. 5,742,379 to Reifer; U.S. Pat. No. 6,369,880 to Steinlechner; U.S. Pat. No. 6,463,393 to Giger; U.S. Pat. No. 6,727,985 to Giger; U.S. Pat. No. 6,859,744 to Giger; and U.S. Pat. No. 6,864,966 to Giger. Although the use of an electrical switch can reduce drift in the electrical portion of an ADM system, it cannot remove drift from the optical portion, which is usually as large or larger than the drift in the electrical portion. In addition, it is difficult to implement an electrical switching system that can switch quickly enough to avoid a phase shift in electrical signals modulated at several GHz. Because of their limited utility and difficulty of implementation, electrical switches are not a good solution for correcting drift in an ADM.
For a bistatic distance meter, there are two references that discuss the use of fiber optic switches. U.S. Published Patent Application No. US2009/0046271 to Constantikes teaches a method in which one fiber switch is placed in the outgoing beam path and a second fiber switch is placed in the returning beam path. These two fiber optic switches are switched at the same time to either permit light from the measure or reference path to reach the optical detector. U.S. Pat. No. 4,689,489 to Cole teaches use of a fiber switch in which light from the return port of the bistatic distance meter is into one port of a switch and light from the outgoing beam is fed into the second port of the switch. The fiber-switch architectures described in these references apply only to bistatic devices and cannot be used with laser trackers for reasons discussed earlier.
There is a need for an ADM that accurately measures moving targets with little drift. It must be monostatic and minimize drift in both optical and electrical components.